This invention relates to a development aid device for a ladder program such as, more specifically, a user program for a programmable controller.
A programmable controller (PLC), which is commonly being used as a control device for factory automation (FA), is typically formed as an appropriate combination of a plurality of units of various kinds such as a power unit for supplying electrical power, a CPU unit for controlling the whole PLC, an input unit for inputting signals from switches and sensors that are set at appropriate positions on a production apparatus or an equipment apparatus for the FA, an output unit for outputting control signals to actuators or the like, and a communication unit for connecting to a communication network.
The control by the CPU unit of a PLC is carried out by cyclically repeating the processes of taking in a signal inputted through the input unit to the I/O memory of the CPU unit (IN-refresh), carrying out a logical calculation based on a user program formed by a preliminarily registered ladder language (calculation execution), writing the results of the calculation execution into the I/O memory and transmitting them to the output unit (OUT-refresh), and thereafter carrying out the so-called peripheral processes.
In the above, the user program is usually a ladder program created by using a development aid device and is downloaded to the CPU unit of the PLC through a communication line. When a problem occurs in the created ladder (user) program, the user displays this ladder program on the display screen of the development aid device, finds the problem point and carries out a repair work. The repaired ladder program is downloaded to the PLC.
The work of debugging a ladder program is carried out by using as the key the operand (such as a contact number or a memory number) of the output command which operated abnormally and repeatedly correcting and editing the ladder program by detecting input commands or output commands related to that output command. The contact number means the terminal number that indicates the terminal position of the terminal block of the input unit or the output unit. The memory number means the address value of the IO data memory in the programmable controller. To a person who does the work of debugging, it is a cumbersome job to detect an input or output command that relates to the output command that has operated abnormally. For example, if there are a plurality of input commands that relate to the output command found to have a problem, all of these plurality of input commands must be checked and the process of finding them out is very troublesome. If there is a portion of the program that includes an input command having the same operand as the aforementioned output command, it is necessary, after correcting the program related to the problem output command, to ascertain that this program also functions normally. In other words, it is extremely troublesome to check all commands to be checked without missing a single one. The problem becomes severer as the ladder program becomes longer.